This is a big mouse

There are (many) big mice.


'many' is not a must to put in this sentense or you can put any other word that are similar to 'many'(can be an exact number).

001：I want to say that 'rat' is singular. Moreover, 'mouse' and 'rat' are two different words although their meaning are the same. If you want to change this from singular to plural, you should write 'mice' as it is the plural of 'mouse' but not change the whole word to another('rat' or 'rats').

仲有都唔知點解咁多人經常性地寫「There have」，我講咁多真係只想澄清返唔係特登捉人錯處話人D咩(因為我覺得自己都唔係好勁)，希望真係對你有幫助啦！！

These are big mice

This --> These
That --> Those

is --> are
am --> are

These are big mice(plural).
big(adj)

There have many big rat.